Wireless communication systems commonly include information carrying modulated carrier signals that are wirelessly transmitted from a transmission source (for example, a base transceiver station) to one or more receivers (for example, subscriber units) within an area or region.
FIG. 1 shows a portion of a single cell of a cellular wireless network system. A base transceiver station 110 provides a wireless connection to a plurality of subscriber units 120, 130, 140. The base transceiver station is generally connected to a network that provides access to the Internet. The cell of FIG. 1 is generally repeated forming a cellular network. The base transceiver station 110 and the subscriber units 120, 130, 140 include one or more antennas allowing two-way communication between the base transceiver station 110 and the subscriber units 120, 130, 140.
FIG. 2 shows a typical transmission chain 200 of a wireless transmitter or transceiver. The transmitter receives a stream of data (Data In) to be transmitted. A transmission signal is modulated by the stream of data by a modulator 210. The modulated transmission signal is typically frequency up converted by mixing the modulated carrier signal with a local oscillator (LO) signal through a frequency mixer 220. The frequency up converted signal is generally amplified by a power amplifier 230 before transmission through a transmission antenna T.
Distortion of the modulated transmission signal by the transmission chain 200 can reduce the effectiveness of signal transmission from the transmitter to a receiver. The power amplifier 230 can cause distortion to modulated transmission signal if the amplitude of the modulated transmission signal is too large, and therefore, reduce the effectiveness of the signal transmission.
FIG. 3 shows a example of a typical modulated transmission signal in which the amplitude of the modulated transmission signal varies with time. The modulated transmission signal typically include peaks 310, 320 which indicate the maximum modulated transmission signal amplitude over the time period of interest. A dashed line 330 indicates an average signal amplitude of the modulated transmission signal. A peak to average ratio (PAR) is defined as the ratio of the peaks of the amplitude of the signal, to the average amplitude of the signal.
Generally, it is desirable to maintain a particular average power level. For example, an average power level can insure a desired signal to noise ratio of the received transmission signal. Therefore, it is generally at the peaks 310, 320 in which the power amplifier 230 causes distortion of the modulated transmission signal.
FIG. 4 shows a typical curve 400 representing a relationship between the input signal amplitude versus output signal amplitude of the power amplifier 230. The curve 400 is typically linear until the output signal power become large enough that the power amplifier 230 begins to saturate. Dashed line 410 roughly designates the point in which the power amplifier transitions from an essentially linear range to a non-linear range. As the input signal amplitude increase past the dashed line 410, the output signal amplitude compresses and no longer linearly increases. Operation of the power amplifier in the non-linear range distorts the modulated transmission signal.
It is clear from FIG. 3 and FIG. 4, that it is desirable to minimize the PAR of the modulated transmission signal. It is also clear that the power level of a modulated transmission signal having a given PAR should be adjusted so that the peaks of the modulated transmission signal do not cause a power amplifier of wireless transmitter to saturate, and therefore, distort modulated transmission signal of the transmitter.
Reduction of the PAR of the modulated transmission signal provides two advantageous features. First, a reduced PAR allows transmission of a greater average transmission signal power level. This provides the advantage of an enhanced signal to noise ratio. Second, a reduced PAR allows for the use of a typically less expensive power amplifier for a given average transmission signal power level. That is, the non-linear region of a power amplifier is typically reached at a lower output power level than for a less expensive power amplifier. Therefore, the reducing the amplitudes of peaks of the transmission signal generally allows for the use of a less expensive power amplifier for a given transmission signal power level.
It is desirable to have an apparatus and method that provides reduction of a PAR of a transmission signal. It is additionally desirable to provide adjustment of the average power level of the transmission signal so that a power amplifier of a transmitter of the transmission signal does not excessively distort the transmission signal.